Huntington disease (HD) is a devastating inherited neurodegenerative disease characterized by chorea, dementia, and premature death. It is caused by a mutation within the gene IT15 that encodes an abnormally long stretch of polyglutamines (polyGlns) within the protein huntingtin (htt). The overall goal of our research is to elucidate the molecular mechanisms by which mutant htt induces neurodegeneration. We have developed a cellular model that recapitulates two key features of Huntington's disease: cultured neurons transfected with mutant htt die in a mutation (polyGln expansion)-dependent fashion and the death is cell-specific, affecting only the enkephalin-positive striatal neurons in culture. Htt is expressed widely in the nervous system, so the basis for neuron-specific degeneration in Huntington's disease is unknown. Using the model, we found evidence that mutant htt must translocate from the cytoplasm into the nucleus to trigger neurodegeneration, but the mechanisms that regulate the translocation are not known. How htt acts in the nucleus to trigger neurodegeneration is also unclear. Preliminary experiments with our model suggest that expanded polyGln tracts may alter htt to promote gene expression in a mutation-dependent and transcription factor-specific manner. However, the molecular basis for the effect of mutant htt on gene expression is unknown. To begin to address these unanswered questions, we propose to use our cellular model to accomplish the following Specific Aims: 1) Characterize the mechanisms of neuron-specific degeneration in Huntington's disease by varying independently the type of neuron in which mutant htt is expressed, the expression level of mutant htt, and the protein context in which the polyGln tract is expressed; 2) Characterize the critical processes by which htt is translocated to the nucleus by characterizing the factors that normally regulate htt localization and by mapping within the gene that encodes it, sequences that function to localize it to the cytoplasm, translocate it to the nucleus, or restrict it to sub domains of the nucleus and; 3) Identify mechanisms by which mutant htt, acting in the nucleus, could trigger neurodegeneration by elucidating the molecular mechanisms by which mutant htt potentiates gene expression mediated by the transcription factor Sp1, and by using microarray technology and inducible expression to identify the earliest changes in endogenous gene expression triggered by mutant htt. Completion of these aims should reveal new insights into general mechanisms of neurodgeneration and to the identification of potential molecular targets for new HD therapies.